BIOLOGICAL PAPERS 1 27 



Our last picture is that of a community containing chiefly the 

 blue flag (Iris i-ersicolor) and the blue violet (Jlola cncnUata). 

 Their rhizomes are short and thick, lie just below or at the soil 

 surface, and form a dense mat. Xevertheless, when one or more 

 square feet of this mat were carefully removed and the soil in 

 the interstices among the rhizomes was taken away, it was esti- 

 mated that the interstices, as viewed from above, constituted 

 from T,^ to 60 per cent of the total. Evidently, then, so far as 

 mere room was concerned, several other species could have grown 

 — in fact, did grow — in these interstices. But they were plants 

 which rooted higher or lower ; or, if at tlie same level, they were 

 species not largely dependent upon rhizomes or stolons for multi- 

 plication. Thus, where the blue flag had reached a maximum of 

 frequency, the water knot-weed, with a low root system, and the 

 bedstraw (Galium Claytoni), with a high root system, might 

 live; but the sweet flag (Acorns calamus), wth rhizomes similar 

 to those of the blue-flag and lying at a similar depth, and de- 

 pendent largely on rhizomes for multiplication, was absent. 



A detailed presentation of the conclusions growing out of my 

 work is impossible here. But in closing I would point out the 

 importance of exhaustive study, in the future, of the inter- 

 relationships among subterranean organs, especially since these 

 have much to do with the compactness of vegetative growth in an 

 association. For. induced directly or indirectly by the condi- 

 tions following such compactness, have doubtless arisen many of 

 the most important adaptations in the groA\i:h forms of various 

 species. 



THE RAXGE OF E\'APORATIOX AND SOIL MOISTURE 



IX THE OAK-HICKORY FOREST ASSOCIATIOX 



OF ILLIXOIS. 



W.\DE m'XUTT and GEO. D. FULLER. 

 I. EVAP0IL\TI0N. 



The oak-hickory forest, an association characterized by the 

 presence of the white oak (Querciis alba), the red oak (O. rubra) 

 and the shag-bark hickorv- (Carya ovata) as its dominant tree 

 members, occupies a unique position in Illinois, appearing as the 

 climax association of much of the woodlands bordering upon the 



